Fix stream-pausing in dart2js.

sdk/lib/async/stream_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // part of dart.async;
 
 // States shared by single/multi stream implementations.
 
 /// Initial and default state where the stream can receive and send events.
 const int _STREAM_OPEN = 0;
@@ skipping 161 matching lines @@
 
   /** Record a decrease in the number of times the listener has paused. */
   void _decrementPauseCount(_StreamListener<T> listener) {
     assert(_isPaused);
     listener._decrementPauseCount();
     _updatePauseCount(-1);
   }
 
   /** Update the stream's own pause count only. */
   void _updatePauseCount(int by) {
-    _state += by << _STREAM_PAUSE_COUNT_SHIFT;
+    int oldState = _state;
+    // We can't just _state += by << _STREAM_PAUSE_COUNT_SHIFT, since dart2js
+    // converts the result of the left-shift to a positive number.
+    if (by >= 0) {
+      _state = oldState + (by << _STREAM_PAUSE_COUNT_SHIFT);
+    } else {
+      _state = oldState - ((-by) << _STREAM_PAUSE_COUNT_SHIFT);
+    }
     assert(_state >= 0);
+    assert((_state >> _STREAM_PAUSE_COUNT_SHIFT) ==
+        (oldState >> _STREAM_PAUSE_COUNT_SHIFT) + by);
   }
 
   void _setClosed() {
     assert(!_isClosed);
     _state |= _STREAM_CLOSED;
   }
 
   void _setComplete() {
     assert(_isClosed);
     _state = _state |_STREAM_COMPLETE;
@@ skipping 816 matching lines @@
     if (_isComplete) {
       throw new StateError("Subscription has been canceled.");
     }
     if (_timer != null) {
       _timer.cancel();
       _timer = null;
     }
     _pauseCount = 0;
   }
 }